The
GSF is the fallacy of supposing that because a group is in the right
formation, it is necessarily on the right course; and conversely, of
supposing that because an individual has strayed from the group and isn't
in formation, that he is off course. The individual may seem lost to the
group but not off course to an ideal observer.

Background:Gadara was the ancient city of Palestine southeast of the Sea of
Galilee and subsequently destroyed. The name was later adopted by a
district east of Jordan and called Gadarenes, or Gergesenes.
It was the site of the famous miracle of the swine, in which Jesus
conjured demonic spirits into the body of swine and let them perish
in the sea. The story is recounted in the Synoptic Gospels.

The
fallacy is elucidated well in R.D. Laing's Politics of
Experience. Here is an excerpt from that work:

From an ideal vantage
point on the ground, a formation of planes may be observed in the air.
One plane may be out of formation. But the whole formation may be off
course. The plane that is 'out of formation' may be abnormal, bad or
'mad,' from the point of view of the formation. But the formation itself
may be bad or mad from the point of view of the ideal observer. The
plane that is out of formation may also be more or less off course than
the formation itself is.

The 'out of formation'
criterion is the clinical positivist criterion.

The 'off course'
criterion is the ontological. One needs to make two judgements along
these different parameters. In particular, it is of fundamental
importance not to confuse the person who may be 'out of formation' by
telling him he is 'off course' if he is not. It is of fundamental
importance not to make the positivist mistake of assuming that, because
a group are 'in formation,' this means they are necessarily 'on course.'
This is the Gadarene swine fallacy.